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Plant Life Cycles, Flowers and Fruits The plant kingdom includes: Nonvascular plants: algae and bryophytes Algae (green and red algae) Single to multicelled Various forms of reproduction, but no seeds Chloroplasts have chlorophylls a and b Cell walls mainly cellulose Starch used as a storage polymer

Spirogyra

Volvox


The plant kingdom includes: Bryophytes (mosses and liverworts) Non-vascular land plants Have root-like rhizoids, but lack true roots Reproduce by spores, not seeds Dominant phase of life cycle is haploid gametophyte Male gametes have flagella, water required for fertilization to form sporophyte Diploid sporophyte is attached to gametophyte Meiosis occurs in sporophyte capsule to produce haploid spores www.botany.ubc.ca/bryophyte/stanleypark


The plant kingdom includes: Vascular plants: Pteridophytes (ferns & fern allies) seed plants (Angiosperms & Gymnosperms) Pteridophytes Vascular land plants Have vascular tissue for conducting water, true leaves, roots Some have lignified cell walls Reproduce by spores, not seeds Diploid sporophyte is dominant phase of life cycle, typical fern plant Haploid gametophyte is small, produces male (sperm) and female (egg) gametes Male gametes have flagella, water required for fertilization to form sporophyte Spores produced in sporangia on leaves of the sporophyte plant Clusters of sporangia are called sori


The plant kingdom includes: Seed plants (spermatophytes): Gymnosperms and Angiosperms Have well developed vascular tissue, fully adapted to life on land Reproduce by seeds Sporophyte is the dominant phase in the life cycle Gametophyte stage occurs in the flowers Male gametes (pollen) do not have flagella like Bryophytes & Pteridophytes Gymnosperms include coniferous trees, cycads, Gingko Gymnosperm gametophytes are the male (pollen) and female (seed) cones Female gametophyte and developing seed not enclosed within a carpel Ovules are fertilized by direct contact by pollen Economically and agriculturally important Pinaceae (wood fiber, resin, some seeds)

Angiosperms include the majority of flowering plants, most cultivated plants More than 250,000 species Angiosperm male gametophytes are pollen grains, only 3 cells Female gametophytes are also reduced, 7 cells Female gametphytes are enclosed within an ovary in a carpel


Gymnosperms

cycad

spruce

gingko


The plant kingdom includes: Angiosperms, the flowering plants Two major groups, monocots and dicots (eudicots) Monocots include the grasses, palms, orchids, lilies Dicots include everything else Characters of monocots

Characters of dicots

Embryo has one cotyledon Embryo has two cotyledons Leaves with parallel veins Leaves with net veins Pollen with a single pore Pollen with three pores Three-part flowers Four or five-part flowers Vascular bundles distributed in stemVascular bundles in central ring No secondary xylem Secondary xylem may be present Fibrous root systems Taproots


carpels and ovules

stamen

stamens and pollen grain

two-celled pollen grain


Flower Structure Flowers evolutionarily derived from branches Four floral organs: Sepals, leaf-like, outermost layer usually green, photosynthetic, but sometimes petal like whorl of sepals called the calyx Petals, modified, nonphotosynthetic leaves often bright colors whorl of petals called the corolla Stamens, male reproductive structures anthers enclose the male gametophyte, pollen anthers supported on a stalk called a filament whorl of stamens called andoecium Gynoecium, the female reproductice structure carpels enclose the female gametophyte, ovules composed of stigma, style, ovary


Angiosperm vs, Gymnosperm Flower Structure In gymnosperms, ovules are exposed on “sporophylls� Sexual fusion between the male and female gametes to form a diploid zygote occurs by direct contact between pollen and the ovule In angiosperms, ovules are enclosed within carpels that become the tissues surrounding the seed (fruit) The complex structures of angiosperm flowers reflect the requirements for accomplishing sexual fusion between the male and female gametes when the female gamete are embedded inside the flower tissue The structure of angiosperm flowers also has facilitated an evolutionary radiation based on animal assisted pollination


Complete and Incomplete Flowers Flowers that have all four components are called complete flowers If any of the four components is absent, the flower is an incomplete flower Flowers that have both the androecium and gynoecium are perfect flowers Flowers with only the androecium or only the gynoecium are imperfect Monoecious plants have both male and female flowers on the same individual Dioecious plants have only male or female flowers on a single individual Flowers of dioecious plants are therefore always both incomplete imperfect Grass flowers lack sepals and petals but have stamens and carpels, so are incomplete, perfect flowers


Flower structure, male and female gametophytes

male gametophyte

female gametophyte


Variations in flower structure Grass flowers are incomplete, perfect

Flowers are classified according to the relative position of the ovary to other flower parts

Regular flowers have radial symmetry Irregular flowers have bilateral symmetry


Male and Female Gametophytes in Angiosperms In Bryophytes and Pteridophytes, the diploid sporophyte and haploid gametophyte phases occur on morphologically distinct plants In angiosperms, the male and female gametophyte are very reduced and occur only within the flower. Meiosis, the cell division that produces haploid gametes, occurs in the corresponding male and female flower structures, the anthers and ovules Male gametophytes, pollen, are derived from microspore mother cells Microspore mother cells are initially diploid, undergo meiosis to produce four microspores Microspores undergo a mitotic division to form the male gametophyte The mature male gametophyte contains two nuclei, a vegetative cell nucleus and a generative cell nucleus


Male and Female Gametophytes in Angiosperms Development of female gametophytes is a bit more complicated One to several ovules (depending on the plant species) are embedded within the ovary The ovule consists of a megasporangium and surrounding tissue layers An opening, the micropyle, exposes the megasporangium at one end

embryo sac ovule

integuments micropyle


Male and Female Gametophytes in Angiosperms The megaspore nucleus undergoes meiosis in the megaspore mother cell Four haploid nuclei result from meiosis, three of these degenerate The remaining nucleus divides several times, depending on the plant species Three mitotic divisions is typical, resulting in eight nuclei one nucleus from each end of the developing female gametophyte (polar nuclei) migrate to the center cell walls form around the egg, synergid and antipodal nuclei egg 2 synergid cells

polar nuclei 3 antipodal cells


The life cycle of an angiosperm showing sporophyte and gametophyte phases


Double Fertilization How does sexual fusion occur between the male and female gametes? The process of fertilization of the female gametophyte in angiosperms is called double fertilization First pollen must be brought into contact with the receptive surface of the stigma In self-compatible plants this may occur by contact of pollen and a stigma in the same (perfect) flower In outcrossing plants, pollen may be transmitted between different individuals by pollinating insects, or through wind pollination (typical of grasses) The surface of the stigma has sucrose and other substances that cause the pollen grain to germinate to form the pollen tube


Double Fertilization The pollen tube grows as a long filament down the style toward the ovule At the same time, the generative nucleus divides mitotically to form a second nucleus to form two sperm nuclei pollen tube sperm nuclei

antipodal cells polar nuclei egg cell synergid cells

vegetative nucleus

micropyle


Double Fertilization When the pollen tube reaches the female gametophyte, it fuses with the cell wall of the embryo sac near one of the synergid cells At this point, the synergid nuclei, the antipodal nuclei, and the vegetative nucleus of the pollen tube all degenerate, leaving the sperm nuclei, egg, and polar nuclei antipodal cells pollen tube sperm nuclei

polar nuclei egg cell synergid cells

vegetative nucleus

micropyle


Double Fertilization One of the sperm nuclei migrates into the female gametophyte and fuses with the two polar nuclei, forming a primary endosperm nucleus The other sperm nucleus fuses with the egg cell to form a zygote

pollen tube polar nuclei

sperm nuclei

egg cell

micropyle


Double Fertilization Because the primary endosperm nucleus is formed from fusion of three haploid nuclei, it has 3 sets of chromosomes, triploid The zygote has two sets of chromosomes, so it is diploid

pollen tube

primary endosperm nucleus

zygote synergid cells micropyle


Double Fertilization The zygote divides mitotically multiple times to form a multicelled embryo The embryo consists of the cotyledons, hypocotyl, shoot apical meristem, root apical meristem The primary endosperm nucleus divides to produce starchy endosperm Endosperm provides a source of carbon and energy for growth of the young plant until it can begin to photosynthesize Endosperm is, of course, the source of starch in cereal grains on which the majority of human civilizations depends Fertilization triggers several developmental changes in the ovary as they enlarge and change to form a fruit


Endosperm Endosperm in seeds may be starch that occupies most of the volume of the seed, as in cereal grains In some plants like legumes, the endosperm is absorbed by the developing embryo and may be converted to lipids and protein, as in dry beans In coconuts, the endosperm is present primarily as lipids


Seeds Seeds are an adaptation by plants to life on land. A seed provides the embryo with a protective case to allow it to survive unfavorable conditions and provides it with a food supply until it can begin to manufacture its own food through photosynthesis. Seeds and fruits may also represent adaptations to life on land by having mechanisms to facilitate dispersal by wind or by animals, and by having mechanisms to survive passage through animal digestive tracts. Gymnosperms and Angiosperms both produce seed, but in Angiosperms the diversity of seeds and the fruits that contain them is highly diverse. The seeds and fruits of angiosperms are the foundation of human food, and the majority of the remainder of this course will be concerned with their various types and the ways humans use them


Seeds What makes seeds a good source of food for humans? High caloric value, contain starch, lipids or protein (legumes) Often produced in large quantities Can be harvested without disturbing the plants that produced them

Plant Lifecycles, Flowers, and Fruits  

Lecture on Plant Lifecycles, Flowers, and Fruits

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